Wednesday, August 19, 2020

Which dingy for S/V Lynx?

Just like our original thoughts on which catamaran to go with, our current choice for a dinghy has evolved over time.  Let's use the way back machine and go back about ten years to when we were thinking about which type of cat we wanted to buy (someday).  

Initially, we were looking at a custom-built, Kurt Hughes designed, catamaran  (see right).  We even had this half-finished one picked out.  (Note, someone else bought and finished her and is now out sailing the world.)  She was 58' long.  

At that time, we thought that a light, narrow hulled, faster cat was just what we wanted!  Then, over time, we slowly came to realize that comfort trumps speed.  With all the people and sports toys we want to have with us, we would over loaded a fast cat, turning it into a slow cat.  Instead, by starting with a cat with wider hulls that is designed for load carrying capacity, we could get almost the same speed out of it as the overloaded (previously) fast cat.  We figure, if we are only going to go at 8 knots anyway, why not have all the space and comforts?  

The more we looked at this, the more our choices changed to the two boats we are currently interested in buying.  (For more on cats, see 'Best Cat for Us' on our website).  However, here is a picture of one of the cat models we are currently interested in buying (see below, left).

Now, our choice in a dingy is going through a similar evolution.  Initially, we were interested in a big tender with seats, a steering console, and a big-assed powerful gas outboard.  Now reality has started to set in.  If we buy such a tender, with the motor, she will weigh in at over 600 lbs, possibly over 700.  There are three major issues with that.  

1) That adds a lot of weight to our catamaran, and though our comfy style 'fat cat' can handle a lot of weight, there are still limits, and the lighter she is, the faster she will go (up to a point).  

2) It puts an extreme amount of weight on the stern.  That isn't good.  The stern is designed to handle a certain amount of load.  Too much will shove the sugar scoops down and cause hobby horsing as we sail.

3) A heavy tender cannot be taken up onto a beach.  It has to anchor out or go to a dock.  There will be many times when we wish to beach our dinghy.

So, that means we have to put our future tender on a diet and lose some significant amount of weight.

Here is the conundrum.  A small dinghy, with a tiny outboard, is nice and light for the catamaran and can be beached easily (see pic right).  It is also frugal on the use of gasoline, which means we don't have to store a lot of flammable and (sometimes) hard to get fuel onboard the mothership.  But, that small light dingy with its tiny outboard can't go great distances at a decent speed.  What happens when we want to load up some dive gear and divers, and then motor to a distant dive site, far from where our main catamaran is anchored?  

For that, we need a more powerful dinghy with some reasonable amount of interior space.  However, that adds too much weight to beach the boat.

We have found a compromise we can live with.  First off, we are going to go with an electric propulsion system.  At first glance, this might seem foolhardy, since gas packs a whole lot more energy by weight than batteries.  This is true.  However, I will get to more about this in a moment.

If we go with a large yet light dingy, we have figured out a way to have our cake and eat it too!  The boat can even have a few comforts, like a twin floor to keep stuff (and your feet) from sitting in water.  If the dinghy is around 12' long, it fits between the sugar scoops.  If the weight is kept down to under 200 lbs, we can drag it up onto a beach (adding dinghy wheels to the transom to help us).  That's the boat only weight, so what about the motor and fuel source?

Let's get back to that electric propulsion idea.   If we go with an 18 KW electric motor (equivalent to a 25 hp gas outboard), we get high torque at low prop speeds.  This helps us push the boat when it is loaded with a lot of weight.  It also has the big benefit of eliminating gas, both on the dinghy and storing extra on the mothership.  We don't have to buy gas, cart it to the boat, store that flammable fuel on board, pour it into the dingy fuel can, or smell it as it burns.   We also don't have to deal with the complexity of a gas engine, including carburetors that occasionally get clogged or a pull start that you keep yanking when your gas engine doesn't want to start.  We get instant power, no starting necessary, with less maintenance.

In exchange, we have to accept a smaller motor and our fuel source will not take us as far at high speed.  Battery power will take you a long distance, but at the cost of speed.  If you want to go fast, then you sacrifice range.  Most of the time that's fine.  We won't often be taking the dinghy for extremely long distances.  With a 48volt battery pack, with 300 amps, Our a 25 hp equivalent electric motor, we should go for about one hour at full speed traveling about 18 miles. 

Just to note, if we cut the speed in half, we can go about six to eight hours instead of one.  If you figure we are going from 18 mph, full out, to about 9 mph, at half speed, you can see how you extend your range.  Full out, you go 18 miles.  At half throttle, you go more like 100 miles. Now, we don't think that will hold completely true, those kinds of simple power calculations don't account for ocean conditions, etc., but if we figure we go even half that, 50 miles, that is still a decent range which is sufficient for our needs.

Best yet, we don't have to buy or store gas anywhere.

Let's get back to talking about how we can have a boat that goes far with a decent amount of people and gear, yet can also be easily beached.  Generally, most of the boats we have seen out there do one of three things.  They have a small dinghy with a tiny motor and give up the gear and people going long distances. Or, they have a bit dinghy with a powerful gas motor, yet give up beaching the boat.  Finally, there are compromise boats, where they are just light enough to beach and just powerful enough to go plain.  The problem with these boats is that as soon as you put much weight in them they no longer plain. 

But, we have the fourth option.  We're going to buy two motors for our fairly light dinghy.  When we want to go to a nearby beach and take the dinghy up on the sand, we will remove the 25 hp electric outboard and the large battery pack and replace the larger motor with a small electric ePropusion Spirit 1.0 Plus, 3 hp electric outboard that has an included self-contained battery (see pic right).  This motor only weighs 43.4 pounds, including the battery.  It is also completely waterproof'; it can be safely submerged in shallow water.  Therefore, if the dinghy gets swamped by a wave while exiting or entering the surf, it doesn't matter.  And, the entire weight of the boat is now under 250 lbs, which will work for beaching the dinghy with transom mounted wheels.

When we need a lot more power and don't need to beach the dinghy, we use the 18 KW motor and the large 13 kWh battery pack.

So, there you have it, a convertible, fairly light dinghy that works for two purposes.  We think it's a good compromise.  As for which dinghy, we're thinking about the ZAR Mini 13 or the Highfield CL380.  For more information on why and which dinghy we are choosing, see our website, or click on this link Tender

The dinghies we are interested in weigh less than 200 lbs, and each of them has a double floor and is rated for six or seven passengers.  This gives us a good size tender with a decently powerful electric motor (for the weight of the boat) as well as a light enough dinghy to get on the beach when we change to the much smaller motor.